Thermoplastic elastomers are known. They have many of the properties of thermoset elastomers, yet they are processable as thermoplastics. One type of thermoplastic elastomer is a thermoplastic vulcanizate, which may be characterized by finely-divided rubber particles dispersed within a plastic. These rubber particles are crosslinked to promote elasticity. Thermoplastic vulcanizates are conventionally produced by dynamic vulcanization, which is a process whereby a rubber is cured or vulcanized within a blend with at least one non-vulcanizing polymer while the polymers are undergoing mixing or masticating at some elevated temperature, preferably above the melt temperature of the non-vulcanizing polymer.
Many conventionally produced thermoplastic vulcanizates employ an ethylene-propylene-diene terpolymer (EPDM) as the rubber of choice. These terpolymers have typically been synthesized by using solution polymerization techniques. A shortcoming of solution polymerization is the inability to synthesize high molecular weight polymer (e.g., Mw of 500,000 or more) without oil extending the polymer product. The same shortcomings exist when slurry polymerization techniques are used. Because the use of high molecular weight EPDM rubber is desirable in the manufacture of technologically useful thermoplastic vulcanizates, oil extended EPDM is often used. And, as a result, the oil that is employed to extend the EPDM ultimately becomes part of the thermoplastic vulcanizate. The ability to select an oil during manufacture of the thermoplastic vulcanizate is therefore limited. This can be disadvantageous because it is often desirable to tailor the performance characteristics of the thermoplastic vulcanizate with various oils.
Furthermore, conventional solution-polymerization techniques produce rubber bales, and these bales are then pre-processed by granulating the rubber prior to manufacture of the thermoplastic vulcanizate. This additional manufacturing step can be energy intensive, time consuming, costly, and involves additional process complications.
Conventionally produced thermoplastic vulcanizates also typically include carbon black. Although carbon black is typically added to the composition prior to dynamic vulcanization, conventional wisdom suggests that the carbon black becomes primarily incorporated into the plastic matrix of the thermoplastic vulcanizate. As a result, the advantages associated with carbon black, such as the UV stability, are not believed to be fully realized in the rubber phase.
Because the number of uses of thermoplastic vulcanizates is increasing, the performance demands that are placed on these materials is more demanding, and the manufacturing efficiency of the materials is continually pursued, there exists a need to overcome some of the shortcomings associated with the prior art materials and methods of manufacture.